Any type of inductive sensor, whether linear or rotary, has the advantage of making it possible to determine the position of a mechanical component, or any other element, without requiring contact with the component whose position it is desired to know. This advantage means that the applications of such sensors are very numerous types of industry. Such sensors are also used in in all consumer applications, for example in the automotive sector, in which the present invention has been made. It may, however, be used in other diverse and varied fields.
There are principally three types of inductive sensor. The first type of sensor relates to linear inductive sensors by which a translational movement of a mechanical component is measured. The second type of sensor relates to rotary inductive sensors, which follow a rotational movement of a mechanical component about an axis, these rotary inductive sensors also been referred to by the term resolver. The term resolver relates to a position sensor which is furthermore capable of taking the measurement even at a high rotational speed. A third type of sensor relates to sensors capable of carrying out a linear as well as rotary measurement function, among which are sensors marketed under the brand name Inductosyn®.
A contactless inductive sensor is a position sensor the principle of which is based on variation of the coupling between the primary and the secondaries of a transformer operating at high frequency and without a magnetic circuit. Such a sensor therefore comprises a fixed part of transformer type with a primary circuit and at least two secondary circuits, the high-frequency alternating current being capable of inducing an electrical voltage in each of said at least two secondary circuits. It is possible for the transformer to simply be a printed circuit on which the primary and secondaries are formed by tracks inscribed on this printed circuit.
The coupling varies as a function of the position of a moving conductive component, which is the seat of induced currents and the position of which with respect to the transformer it is desired to know precisely, this component forming the second part, this second part being the only moving part of the inductive sensor and being referred to as the target. The inductive sensor is referred to as being contactless because there is no contact between the fixed and moving parts of the inductive sensor.
The primary circuit is supplied by an external source varying as a function of time at high frequency, and the secondary circuits are the seat of induced voltages. The conductive target whose position it is desired to know generally has a simple shape. For a linear sensor designed to measure a translational movement of a mechanical component, the target has a parallelepipedal shape, and for a rotary sensor the target comprises an angular sector with a given angle. The dimensions of each of these targets, as well as the elements of the primary circuit and secondary circuits, must be selected in order to optimize the characteristics of the specifically linear or rotary sensor.
The fluxes of the secondaries, divided by the flux of the primary, form, as a function of the position of the target, envelopes of precise values with a given amplitude, which are independent of time. The values of the fluxes of the secondaries and of the primary are advantageously set up to produce sine and cosine functions of the position of the target over the entire travel of the sensor.
These sine and cosine functions are very useful in the electronic processing of the sensor. The ratio of the two functions is calculated before taking the arctangent, the result of the arctangent thus giving an image of the position of the target. The argument of the sine and cosine functions is a linear or affine function of the position of the target. Thus, the travel of the target represents a more or less large part of the spatial period of these trigonometric functions. The behavior of this sensor is therefore identical to that of a transformer with an emitting primary winding and two secondary windings. From a physical point of view, the modification of the coupling of the primary with the secondaries takes place by means of the electromagnetic skin effect.
Since the primary is supplied with high frequency, the phenomena occurring in the sensor are therefore all at high frequency. The target, which is a solid conductive component, is therefore the seat of large induced currents. The penetration depth of these induced currents is given by the conventional formula for the skin depth. In view of the fact that the calculation gives a value of 50 μm for aluminum, which is a favored but nonlimiting material for the target, the induction therefore penetrates almost not at all into the target. The magnetic flux produced by the primary therefore goes around the target. This greatly modifies the field lines. This modification is seen by the secondaries, which receive more or less flux depending on the position of the target. These fluxes which are variable depending on the target are also variable as a function of time. They therefore generate a voltage across the terminals of the secondary circuits, which is measured by the electronics. The sensor therefore measures the position of the target as a function of the fluxes received in the secondary circuits.
Document US-A-2014/167788 describes a rotary inductive sensor for measuring the angular position of a mechanical component in rotary movement, having a primary winding associated with at least one secondary winding, and a target attached to the component in rotation about a central axis. The primary winding is centered around an axis coinciding with the central axis of rotation of the target, and is flowed through by a high-frequency alternating current capable of inducing an electrical voltage in each secondary winding. The target consists of a plurality of angular sectors with the same angular aperture, which are distributed evenly at one end of the mechanical component in rotary movement.
It is known that inductive sensors, particularly rotary inductive sensors, in which the value measured for the target is an angle, exhibit errors in the measurement of the angular position of the target, therefore of the mechanical component in rotary movement. In this document, it is proposed to reduce the angular aperture of each angular sector of the target by an adjustment angular sector, in order to eliminate one order of the harmonics in a Fourier expansion of the deviation from linearity. This requires a transformation of the complicated angular sectors and only partially solves the problem of the errors in the angular position measurement of the target for a rotary inductive sensor.
Furthermore, the target of the rotary inductive sensor is frequently positioned at one of the two ends of the mechanical component, which is often in the form of a shaft. Particularly in the field of motor vehicles, it is common that the two ends of such a shaft are fixed to other components and cannot accommodate the angular sectors of the target.